A membrane separation technique, which makes use of the difference in permeability of membranes to different gases, is known for separating a gas mixture containing at least two different gas species into the individual gas species. According to this method, the permeate gas or the retentate gas can be recovered to give a target gas either as a high purity, high-permeability gas or a high purity, low-permeability gas.
The permeability of a membrane, which is the volume of permeation, through the membrane, of each gas contained in the gas mixture per unit membrane area, per unit time, and per unit partial pressure difference, can be expressed as P′ (unit: ×10−5 cm3(STP)/cm2·sec·cmHg). The gas selectivity of a membrane can be expressed in terms of the ratio of the permeability to the high-permeability gas to the permeability to the low-permeability gas (i.e., permeability to high-permeability gas/permeability to low-permeability gas).
Generally speaking, a membrane with high gas selectivity has a low gas permeability (particularly to a high-permeability gas), and conversely, a membrane with a high gas permeability (particularly to a high-permeability gas) has a low gas selectivity. Thus, in the case of recovering a low-permeability gas from a gas mixture by using a single-stage gas separation membrane, the use of a membrane with high gas selectivity achieves a high recovery rate if the purity of the gas to be recovered is constant. In that case, however, seeing that the permeability is low, it is necessary to increase the membrane area or increase the operating pressure. Although a membrane having a high-permeability, on the other hand, does not require an increase in membrane area or an increase in operating pressure, the achievable recovery rate is low because of the low gas selectivity.
In general, a gas separation membrane is used as incorporated in a gas separation membrane module wherein the gas separation membrane having gas permselectivity is housed in a casing having at least a gas inlet port, a permeate gas discharge port, and a retentate gas discharge port. The gas separation membrane is mounted in the casing in a manner such that the space on the gas feed side and the space on the gas permeation side are separated from each other. In order to obtain a predetermined membrane area, a gas separation system usually includes a gas separation membrane unit having a plurality of such gas separation membrane modules combined in parallel. The plurality of gas separation membrane modules constituting the gas separation membrane unit share the gas inlet port, the retentate gas discharge port, and the permeate gas discharge port, and thus, the gas separation membrane unit functions substantially as a single gas separation membrane module having a large membrane area.
A multi-stage gas separation system including a plurality of gas separation membrane units is known for recovering, as a target gas, low-permeability gas with high purity at a high recovery rate. When a membrane having a high gas permeability is used, the amount of the gas flowing in the system increases due to the low gas selectivity. In that case, the power consumption for gas compression increases in order to maintain desired purity and recovery rate. In the case of using a membrane having high gas selectivity, although the gas compression power reduces, the membrane area should be increased on account of the low permeability to a high-permeability gas.
As regards multi-stage gas separation membrane units, Patent Document 1 below, for example, proposes a gas separation method using three gas separation membrane units. According to this method, a gas mixture to be separated is fed to a gas compressor, from which a compressed gas mixture is fed to a first gas separation membrane unit, and the retentate gas discharged from the first gas separation membrane unit is fed to a second gas separation membrane unit, while sending the permeate gas discharged from the first gas separation membrane unit to a third gas separation membrane unit. The permeate gas discharged from the second gas separation membrane unit and the retentate gas discharged from the third gas separation membrane unit are returned to the suction side of the compressor.